Gravity Hill is something from the late 1990's. I heard one of the Antonito guys call it that. I guess it stuck from there.
The name is odd in that there is less grade climbing up the side of the mesa than on adjoining stretches. The grade eases a bit from the 1.42% around MP 488, then once you get into the part on the side of the mesa, it eases off further. The easy pull, continues around the curve at the top of the mesa at 290. There, on what seems to be flat ground, the grade stiffens back to 1.42% to where Lava siding was, then begins to ease off again for the run around Lava Loop.
There is an optical illusion here as the entire area slopes toward Mt Blanca at about 1.42%. When you approach the mesa, it slopes down grade away from you while the track climbs up grade. It looks like the grade stiffens to about 3%. I remember my first firing trip west when I saw this and thought "oh crap, I need to get busy". So, I loaded 487 up with coal expecting the engineer to widen out on the throttle. He didn't and we continued our sedate shuffle with a big cloud of black smoke, followed by the pops lifting. The engineer, who was a retired Rock Island hogger Scenic Rys hired for the summer looked over at me, smiled and said "don't worry, I did the same thing on my first trip. Thought we going to get a real work-out here".
The point being there is no more "gravity" (ie grade) here than anyplace else, in fact there is less.